Crop plants have been developed over the last 10 000 years and for most of this time they were not heavily fertilized. However, in the last 50 years the nitrogen fertilization of crop plants worldwide has increased more than 20-fold. The use of this fertilizer is generally inefficient with only about 50% being recovered in the harvested crop. Crop plants did not evolve under conditions of high nitrogen nutrition and many of the mechanisms are not necessarily suited to growth under such nutrition. The question therefore arises, can we, based on our knowledge and the experimental techniques now available to us, improve the efficiency of nitrogen use by crop plants? Two ways appear possible, one to make best use of the available variation in nitrogen use characteristics within the gene pool and, the second, to try to introduce new genes, which might increase that variation.
Increased nitrogen utilization efficiency by plants would have a number of beneficial effects. For example, nitrogen utilization efficient plants would be able to grow and yield better than conventional plants in nitrogen poor soils. The use of nitrogen efficient plants would reduce the requirement for the addition of nitrogenous fertilizers to crops. Since fertilizer application accounts for a significant percentage of the costs associated with crop production, such a reduction in fertilizer use would result in a direct monetary savings.
A reduction in fertilizer application would also lessen the environmental damage resulting from extensive nitrogenous fertilizer use. These detrimental effects of nitrogenous fertilizer use on the environment are manifested in increased eutrophication, acid rain, soil acidification, and the greenhouse effect.